1. Field of the Invention
This invention relates to an apparatus and methods for continuous separation of magnetic particles from a slurry.
2. Description of the Prior Art
There are a number of circumstances where it would be desirable to continuously separate magnetic particles from a slurry. One example is separation of magnetic catalyst particles from a slurry in a slurry reaction system, such as a slurry phase Fisher-Tropsch synthesis wherein carbon monoxide and hydrogen are reacted to form hydrocarbons in the presence of an iron catalyst. The reactor contains a three phase mixture of gases and a slurry of fine catalyst particles in the hydrocarbons. The success of the iron-based process depends on the ability to economically separate the catalyst from the hydrocarbon products, which often comprise viscous waxes. Another example is separation of magnetic particles from oil which is produced while milling steel such as in production of sheet steel in a rolling mill where oil is constantly sprayed onto the rollers resulting in a slurry of iron particles in the oil. Removal of the iron particles would allow for more efficient use of the oil. Other applications are to be found in separation of: chemical compounds or magnetically doped beads in drug and medicine manufacturing, catalysts used in the petroleum chemicals industries, contaminants in soil remediation, and magnetic components of industrial off-gases. Generally, the technology can be used in a wide range of applications in the mining industry where processing and recovery of particles in the 5 to 100 micron size range is problematical using state-of-the-art magnetic separation as discussed in “Mineral Processing Technology Roadmap,” U.S. Department of Energy, Office of Industrial Technologies, September, 2000.
In the past, solids separation in slurry phase Fisher-Tropsch systems has involved complex mechanical filtration systems performed outside the slurry reactor. Other slurry reaction systems have used batch operated mechanical filtration elements within the slurry reactor to remove catalytic particles from the slurry. In U.S. Pat. No. 5,520,890, Loretzen teaches a Fischer-Tropsch reactor containing a number of vertical reaction tubes, each vertical tube having a filter element (typically a screen) at the top of each tube to produce a filtrate zone above the filter element and retain the catalyst particles within the tube. A heat transfer fluid is circulated in the volume outside of the tubes to remove the exothermal heat of reaction. This system relies on controlling pressure variations across the filter element and fluid communication between a gas space above the filtrate zone and any gas space above the slurry in the tubes to prevent buildup of particles on the filtration elements and resultant need for back flushing to remove the solids and unplug the elements. This reactor has the difficulty of added operation constraints to control the filter elements and need for occasional back flushing.
The use of magnetic separation devices for removing magnetic particles from a slurry is well known. One method is High Gradient Magnetic Separation (HGMS) such as disclosed by Kolm, U.S. Pat. No. 3,676,337 wherein particles are attracted to and trapped in a magnetized filter element containing a matrix such as steel wool based on high magnetic field gradients created by the magnetized matrix. At some point the capacity of the matrix is reached and it is necessary to turn off the magnetic field and flush the particles out with a flowing liquid. A variant on the Kolm HGMS is U.S. Pat. No. 3,902,994 to Marston, which discloses a plurality of matrix containing elements on a carousel, so that one or more elements can be cleaned while another is in service.
An example of an external magnetic filtration system used in a slurry Fisher-Tropsch system was disclosed by Brennan in U.S. Pat. No. 4,605,678, in which two or more alternating high gradient magnetic separators were used to remove iron catalyst from the wax product. Slurry was passed through one of the separators wherein the high gradient magnetic field caused catalyst particles to be retained in a magnetized steel wool filtration element. In this process, the filtration was of necessity batch with respect to the high gradient magnetic separators. Periodically, the catalyst was removed from the filtration elements by demagnetizing the element and back flushing with a liquid so that catalyst could be recycled. During back flushing another separator was used. High gradient magnetic separation was quite effective in removing the catalyst particles, removing over 99% of the catalyst. However, the use of a batch filtration process is undesirable in an otherwise continuous reaction system. In particular, the concentration of catalyst in the slurry (20 or 30 wt. percent) is higher than desirable as a feed-stock for high gradient magnetic separation which is best used as a polishing separator with concentrations of the magnetic fraction of the feed in the 2% to 3% range. Conventional HGMS methods are restricted when the concentration of magnetics in the feed exceeds this range, because of excessive capture-matrix loading which leads to poor non-magnetics weight yields. This effect has been discussed in the following paper which is hereby incorporated herein by reference: R. R. Oder, “The Application of High Field and High Gradient Methods to the Magnetic Separation of Mineral Matter from Micronized Coal,” Separation Science and Technology. 19 (11&12), pp. 761-781, 1984-85. Application of carousel as described in Marston (U.S. Pat. No. 3,920,543) and reciprocating as discussed in “Coal Preparation Using Magnetic Separation,” Volume 2, CS-1517, Volume 2 Research Project 980-2 Prepared by Magnetic Corporation of America, Z. J. J. Stekly, Principal Investigator, which are hereby incorporated herein by reference, show that HGMS units will be problematical for the Fischer-Tropsch application because of the complexity of the apparatus and incompatibility with high temperature and high pressure.
Another type of magnetic separator separates particles in a slurry from each other according to their susceptibility or charge as opposed from removing particles from the liquid in the slurry as in the HGMS separators. This type of separator is useful for selective separation of components of complex particle systems. Such devices are often in the form of an elongated housing with a slurry feed at the top and a number of outlet channels at the bottom for collection of slurries concentrated with solids of differing magnetic properties. One such device, disclosed by Kelland in U.S. Pat. No. 4,261,815 utilizes a plurality of ferromagnetic rods located within the housing and oriented along the axis of the housing. Downstream of each rod are four channels for collection of slurries relatively concentrated in particles having different magnetic properties. Another variant by Kelland is U.S. Pat. No. 4,663,029 in which a single rod is located outside and adjacent to an elongated canister parallel to the axis of the canister. The canister has an inlet at the top and a plurality of outlet ports at the bottom for collection of particles according to their size and magnetic susceptibility.
A somewhat related separator is disclosed by Stelzer in U.S. Pat. No. 5,169,006, wherein a magnetic separator comprises an elongated housing with flow inlet at one end and a number of channels at the other end, with a three dimensional array of rods within the housing which are perpendicular to the direction of flow. The rods are comprised of alternating ferromagnetic and non-ferromagnetic sections. An improvement on the above separator was disclosed by Stelzer in U.S. Pat. No. 5,909,813 and U.S. Pat. No. 6,193,071 wherein a magnetic separator comprises an elongated housing with a feed slurry inlet at one end and multiple outlets at the other end. Inside the housing a plurality of ferromagnetic rods are disposed at an angle between parallel and perpendicular with the rods terminating at opposite sides of the housing. A force gradient is created between adjacent rods by providing an external magnetic field with a field direction parallel to the direction of slurry flow. In one variant triangular cross section rods were employed. Particles are deflected into the different channels by the rods.
In U.S. Pat. No. 5,868,939 Oder disclosed a continuous magnetic separator for breaking emulsions of immiscible liquids by magnetostatic coalescence. One embodiment of that device comprises a vertical separator means having an inlet below a top and above a bottom, outlets at the top and bottom for separately withdrawing a continuous phase and a coalesced phase containing a magnetic additive and a magnetic field in a horizontal plane perpendicular to the direction of flow. The above device may comprise vertical magnetic rods onto which the magnetically doped dispersed phase coalesces and flows to the bottom of the separator where there is an interface between the continuous phase and the coalesed magnetically doped dispersed phase. Thus the use of this separator is limited to separation of emulsified liquid phases and the means for controlling the outlet flows was limited to controlling the depth of a “magnetic plug” of the coalesced phase as would apply to a liquid-liquid separation.
The existing art for carrying out slurry phase reactions, such as Fischer-Tropsch synthesis conducted using magnetic catalyst particles, shows a need for improved methods and apparatuses which provide for continuous separation of relatively finely divided magnetic catalyst particles from the liquid component of the slurry. In a continuous slurry reactor system, a particularly desirable form of separation would be a device which acts as both a continuous clarifier and thickening device wherein a slurry stream would be continuously separated into a substantially catalyst free liquid product and a concentrated catalyst slurry. The slurry would be recycled to the reactor to recover catalyst. Those skilled in the art will appreciate that continuous flow separations are greatly preferred to batch and semi-batch separations such as filtration. An ideal device would look very much like a continuous settler, though those skilled in the art will appreciate that sedimentation is not generally practical for separating fine particles in a pressurized system since the separation rates are often be too low resulting in the need for too large a pressurized sedimentation vessel to be practical.
There is a similar need for a continuous magnetic separator for separation of finely divided magnetic particles from liquids such a lubrication oil so as to continuously produce a clarified oil and a thickened slurry of magnetic particles from a feed slurry.
A still further need is for a truly continuous magnetic separation apparatus and method which could be incorporated within a slurry reactor involving a slurry comprising magnetic particles.